Polymers are used in a wide range of applications due to their stability, elasticity, lightweight, strength, ease of fabrication and formulation, and low cost. These applications include packaging, housewares, buildings, highway construction, insulation (sound, vibration, or heat), ground coverings for agricultural weed and erosion control, adhesives, coatings for controlled release products, absorbents, and the like.
Environmental concerns have suggested a need for materials having polymer-like properties but without the degree of permanence typically associated with synthetic polymers. The decreasing availability of landfill space, as well as the increased costs of municipal solid waste disposal, have put increasing emphasis on minimizing the impact of nondegradable materials, including plastics, on the solid waste stream. Man-made polymers are typically not readily degraded by microorganisms that degrade most other forms of organic matter and return them to the biological life cycle. Although synthetic polymers form a relatively small fraction of the materials in landfills today (about 7% by weight or 15-20% by volume, see Thayer, Chem. Eng. News. 1989, 67 (4), 7), it would nonetheless be desirable to formulate such materials so they would be sufficiently durable for their intended use but more susceptible to environmental degradation. This would facilitate the development of methods such as industrial composting to convert municipal solid waste materials to useful products. In addition, plastic film products applied to the ground (e.g. to control weeds and/or erosion) would ideally be formulated to degrade after a few months. Improved degradability would also be desirable for "controlled release" of an active from some products, such as encapsulated pesticides, herbicides, and fertilizers.
Several approaches to enhance the environmental degradability of polymers have been suggested and tried. These include: (1) incorporation of a particulate biodegradable materials such as starch; (2) introduction of photodegradation-sensitizing groups into the molecular structure of the polymer; (3) incorporation of small amounts of selective additives that accelerate oxidative and/or photo-oxidative degradation. Each of these methods has certain problems. The inclusion of starch in polymer compositions facilitates mechanical breakdown, but leaves behind residual components of the nonbiodegradable polymer. Photodegradation functions only if the plastic is exposed to light (e.g., in the case of litter), and provides no benefit if the product is disposed of in a dark environment, e.g., in water, soil or a standard landfill. Oxidative accelerators can cause unwanted changes in the mechanical properties of the polymer, such as embrittlement, prior to or during use.
Another approach to environmental degradability of articles made with synthetic polymers is to make the polymer itself biodegradable or compostable. Biodegradation typically refers to the natural process of a material being degraded under anaerobic and/or aerobic conditions in the presence of microbes, fungi and other nutrients to carbon dioxide/methane, water and biomass. Composting typically refers to a human controlled process (e.g., a municipal solid waste composting facility) where the material undergoes physical, chemical, thermal and/or biological degradation to carbon dioxide/methane, water, and biomass Composting is generally conducted under conditions ideal for biodegradation to occur (e.g. disintegration to small pieces, temperature control, inoculation with suitable microorganisms, aeration as needed, and moisture control).
There are a number of polymer-based products for which biodegradability and/or compostability would be desirable. For example, films used in packaging, as backsheets in diapers, and agricultural ground covering are not intended to survive intact for long periods of time. Latex-type polymer products used in binders and adhesives, as well as in paints and coatings, often serve protective roles where stability to the environment is desired. However many products containing latexes are ultimately disposed of in the municipal solid waste stream. These include nonwoven products (e.g., tissue/towel products) where latex binders are used to join discrete fibers into a cohesive web. Although many nonwovens contain degradable cellulosic fibers (e.g. rayon), the latex binder is typically non-degradable, e.g., acrylate latexes. Accordingly, it would be desirable to be able to make polymeric latexes, including those useful as binders for nonwovens, that would be biodegradable or at least compatible with other means of disposal of waste, including standard industrial and municipal solid waste composting operations
Another approach to environmental degradability of articles made with synthetic polymers is to make the polymer itself biodegradable or compostable. See Swift, Acc. Chem. Res., 1993, 26, 105-110 for a general overview on biodegradable polymeric compositions. Most of this work has been based on hydrolyzable polyester compositions, chemically modified natural polymers such as cellulose or starch or chitin, and certain polyamides. See, for example, U.S. Pat. No. 5,219,646 (Gallagher et al), issued Jun. 15, 1995 (hydrolyzable polyester). Polyvinyl alcohol is the only synthetic high molecular weight addition polymer with no heteroatom in the main chain generally acknowledged as biodegradable. See also Hocking, J. Mat. Sci. Rev. Macromol. Chem. Phys., 1992, C32(1), 35-54, Cassidy et al, J. Macromol. Sci.--Rev. Macromol. Chem., 1981, C21(1), 89-133, and "Encyclopedia of Polymer Science and Engineering," 2nd. ed.; Wiley & Sons: New York, 1989; Vol. 2, p 220. (Limited reports add poly (alkyl 2-cyanoacrylates) to this list of biodegradable synthetic polymers.)
Natural rubber (cis-1,4-polyisoprene) is also readily biodegradable. Natural rubber retains carbon-carbon double bonds in the main polymeric chain that are believed to facilitate attack by either oxygen and/or microbes/fungi, leading subsequently to chain scission, molecular weight reduction, and eventually total degradation of the polymer. See Heap et al, J. Appl. Chem., 1968, 18, 189-194. The precise mechanism for the biodegradation of natural rubber is not known. Enzymatic and/or aerobic oxidation of the allylic methyl substituent may be involved. See Tsuchii et al., Appl. Env. Micro. 1990, 269-274, Tsuchii et al., Agric. Biol. Chem., 1979, 43(12), 2441-2446, and Heap et al, supra. By contrast, nonbiodegradable polymers such as polyethylene, polypropylene, polyvinyl chloride, polyacrylonitrile, poly(meth)acrylates and polystyrene have saturated carbon-carbon backbones that do not facilitate attack by either oxygen and/or microbes. This biodegradability has been recognized only for the natural form of rubber. See Tsuchii et al., supra, which reports: "Synthetic polyisoprenes, however, were not degraded completely by the organism." More recently, it was reported that synthetic "cis-1,4-polyisoprene does not undergo specific biodegradation." See Kodzhaeva et al., Intern. J. Polymeric Mater., 1994, 25, 107-115.
Unfortunately, natural rubber is biodegradable to the extent that it is too unstable for most uses. Natural rubber also suffers from poor mechanical properties (e.g., strength, creep resistance). Indeed, stabilizers, fillers, and/or crosslinking agents are routinely added to natural rubber to enhance its mechanical properties. Crosslinkers are typically required in order to provide sufficient mechanical integrity for practical use. However, the most common crosslinking process creates a polysulfide linkage, i.e., by vulcanization, that virtually eliminates the biodegradability of natural rubber. See Tsuchii et al. J. Appl. Polym. Sci., 1990, 41, 1181-1187.
Polymeric foams are widely used in many areas where biodegradability and/or compostability would be an asset. In addition to foamed plastics for containers and packaging (e.g., foamed polystyrene), polymeric foams have been used as absorbents in absorbent articles such as diapers and catamenial products. See, for example, U.S. Pat. No. 4,029,100 (Karami), issued Jun. 14, 1977, that discloses a shape-retaining diaper that can employ a foam element in the crotch area of the absorbent pad assembly in order to provide high wet resilience. Certain types of polymeric foams have been used in absorbent articles for the purpose of imbibing, wicking and/or retaining aqueous body fluids. See, for example, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad for diapers and the like where the primary absorbent is a hydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov. 19, 1985 (body fluid absorbing cellular polymers that can be used in diapers or catamenial products); U.S. Pat. No. 4,740,520 (Garvey et al), issued Apr. 26, 1988 (absorbent composite structure such as diapers, feminine care products and the like that contain sponge absorbents made from certain types of super-wicking, crosslinked polyurethane foams).
The use of absorbent foams in absorbent articles such as diapers can be highly desirable. If made appropriately, open-celled hydrophilic polymeric foams can provide features of capillary fluid acquisition, transport and storage required for use in high performance absorbent cores. Absorbent articles containing such foams can possess desirable wet integrity, can provide suitable fit throughout the entire period the article is worn, and can minimize changes in shape during use (e.g., uncontrolled swelling, bunching). In addition, absorbent articles containing such foam structures can be easier to manufacture on a commercial scale. For example, absorbent diaper cores can simply be stamped out from continuous foam sheets and can be designed to have considerably greater integrity and uniformity than absorbent fibrous webs. Such foams can also be molded into any desired shape, or even formed into integral, unitary diapers.
Currently, the absorbent core of most disposable diapers comprises a cellulosic fibrous matrix in which is dispersed particulate absorbent polymers often referred to as "hydrogels", "superabsorbents" or "hydrocolloid" materials. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972; U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972; U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987; and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990. Absorbent cores comprising this cellulosic fibrous matrix and dispersed particulate absorbent polymers are typically at least about 80% compostable in standard industrial and municipal solid waste composting operations.
Making useful absorbent polymeric foams that are completely biodegradable is not straightforward. Many of the conventional polymers, such as the aromatic polyurethanes, that have been used in the past as absorbent foams are not biodegradable, are very poorly biodegraded, or may not be compatible with all municipal solid waste disposal methods (landfill, incineration, composting). Generally the stability of the covalent bonds of polymers previously used in such foams is too great, or suitable extracellular enzymes to attack these polymer structures are unavailable, or the polymeric subunits are themselves not biodegradable and/or compostable.
Accordingly, it would be desirable to develop a biodegradable/compostable polymer that would be useful in making such films, adhesives, fibrous elastics, and absorbent foams. It would be particularly desirable to be able to make an absorbent polymeric foam that: (1) is sufficiently stable under normal storage conditions and in the presence of aqueous body fluids, such as urine, to be useful as an absorbent structure in disposable absorbent articles such as diapers, adult incontinence pads or briefs, sanitary napkins and the like; (2) has adequate and preferably superior absorbency characteristics, including capillary fluid transfer capability, so as to be desirable in high performance absorbent core structures; (3) is compatible with all means of disposal of waste, including standard industrial composting operations; and (4) is sufficiently flexible so as to provide a high degree of comfort to the wearer of the absorbent article.